1. Field of the Invention
The present invention relates to a photovoltaic device for converting optical energy into electric energy and more particularly to an improvement in the resistance of the device to a degradation of the conversion efficiency.
2. Description of the Prior Art
A typical conventional photovoltaic cell such as described in U.S. Pat. No. 4,064,521 comprises a front transparent electrode layer, amorphous silicon (a-Si) layers forming a p-i-n junction structure, and a back electrode layer stacked in that order on a glass plate. The a-Si layers are generally deposited by a plasma CVD (chemical vapor deposition) method, utilizing a gas of a silicon compound.
A photovoltaic cell disclosed in U.S. Pat. No. 4,385,199 is improved in its conversion efficiency by providing a p-layer of amorphous silicon carbide (a-Si.sub.x C.sub.1-x) instead of thep-layer of a-Si. Since the p-layer of a-Si.sub.x C.sub.1-x is optically less absorptive than the p-layer of a-Si, more light irradiation reaches the i-layer of a-Si, which substantially contributes to the photoelectric conversion, which thus becomes more effective due to the so-called window effect whereby the output current is increased.
However a, photovoltaic cell comprising amorphous semiconductor layers of silicon derivatives is degraded in its conversion efficiency after exposure to intensive light for a long time period. The present inventors have reported in Technical Digest of the International PVSEC-1, Kobe, Japan, 1984, pp. 213-216 that such degradation depends strongly on the concentration of an oxygen impurity introduced into the amorphous semiconductor layers, i.e., the degradation ratio decreases with a decrease of the oxygen concentration. With a usual CVD apparatus, however, a small amount of leakage of oxygen from the atmosphere is inevitable, so that it is difficult to completely avoid an oxygen impurity. In order to effectively decrease the oxygen impurity, an expensive and complicated CVD apparatus provided with an ultra high vacuum chamber will be required.